Recent advances in the art of drilling include the use of multiple high-angle development wells and directional and horizontal drilling techniques. High angle drilling techniques involve drilling a well into a discovered earth formation with the drillstring inclined at a substantial angle from vertical. Directional drilling involves drilling a first borehole leg, a transition zone, and a second borehole leg inclined at a substantial angle from the first borehole leg, so as to interpenetrate and exploit multiple oil-bearing sands from a single bore. For example, in horizontal drilling, a first leg may be vertical, and a second leg may be substantially horizontal, with a transition zone therebetween. The transition zone at which the two legs of the borehole meet may range from a gradual curve to an abrupt bend. The severity of the transition zone is measured in either bend radius or angle of inclination per horizontal distance. Thus, a transition zone or dogleg curving at 2.degree.-6.degree./100 ft. (3000-1000 ft radius) is regarded as a "long radius" borehole, whereas a transition zone of 1.5.degree.-3.degree./ft. (40-20 ft. radius) is regarded as a "short-radius" borehole.
High angle, directional and horizontal drilling of wells traversing earth formations offers promise as a way to increase production by changing the geometry of the well from primarily vertical to horizontal. For example, in some locations, such as the mid-field area of the Prudhoe Bay field, the oil column lies between an expanding gas cap and a water leg. Conventional wells begin producing gas and water early and continue with decreasing oil fractions throughout their life. Because the produced gas and water must be reinjected, the overall production rate of the field will soon be limited by the gas handling capacity. Thus, such wells rarely meet their potential with regard to hydrocarbon recovery and production rates. A horizontal completion, strategically placed between the gas cap and water leg, will offer the maximum standoff from both. The resulting benefits are increased production rates at lower drawdown pressures, and consequently, reduced gas and water coning.
Because there is such strong economic incentives to perfect high angle and horizontal drilling, much work is presently being done on such techniques. While horizontal drilling has met with some success, there are still drawbacks which must be considered. The nature of the transition zone of the well has a great deal of influence over the cost of the well and the technical problems associated with it. For example, while it is difficult, but economical, to drill and case a well with a curved section having a "long radius", it is often uneconomical to drill and case a "short radius" well. As the radius of curvature becomes smaller, the greater the difficulty and cost of drilling and casing. For example, conventional casing tubulars have a tendency to buckle in curved sections of boreholes. This makes it difficult for drillstring tools and production tubulars to be passed through the casing, or in severe buckling, for the well section, upstream, to be produced at all.
Thus, there exists a need in the art for improved techniques and equipment for enhancing the economic potentials of high angle and horizontal drilling.